Fredo may have had something to do with either providing cover for the would be assassins or even in their murders because when Deanna is taken from her house she is screming they were "right outside my window."

As for Pentangeli, I don't think Mike ever really suspected him. Right after Pentangeli has his rant at Mike, Neri specifically asks Mike whether there is anything he should do about Frankie, and Mike says let him go back to New York, he's an old man who had too much wine.